Step 1: Prepare the Cabbage: Bring a large pot of salted water to a boil. Carefully place the whole cabbage head into the boiling water. Blanch for 5-10 minutes, or until the outer leaves are tender enough to peel off. Gently remove the outer leaves one by one, using tongs. Repeat the blanching process for inner layers if needed, removing leaves as they soften. Trim any thick veins from the base of each leaf to make them easier to roll. Set aside.
Step 2: Make the Filling: In a medium skillet, heat 2 tablespoons of olive oil over medium heat. Add the finely chopped onion for the filling and sauté until softened, about 5 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 3: Season the Filling: Stir in the cooked lentils, cooked rice, ground cumin, smoked paprika, and ground coriander. Season generously with salt and pepper to taste. Cook for 2-3 minutes, stirring to combine all ingredients and warm through. Remove from heat and let cool slightly.
Step 4: Prepare the Tomato Sauce: In a saucepan, heat 1 tablespoon of olive oil over medium heat. Add the finely diced onion for the sauce and cook until translucent, about 5-7 minutes. Add the minced garlic and cook for 1 minute more.
Step 5: Simmer the Sauce: Stir in the crushed tomatoes, dried oregano, and optional sugar. Season with salt and pepper. Bring to a simmer, then reduce heat to low and let it cook gently while you roll the cabbage.
Step 6: Assemble the Cabbage Rolls: Lay a cabbage leaf flat. Place about 2-3 tablespoons of the lentil and rice filling near the stem end of the leaf. Fold the sides of the leaf over the filling, then roll tightly from the stem end up, creating a neat parcel.
Step 7: Arrange and Bake: Pour about half of the prepared tomato sauce into the bottom of a 9x13 inch baking dish. Arrange the assembled cabbage rolls snugly in a single layer over the sauce. Pour the remaining tomato sauce over the rolls, ensuring they are mostly covered.
Step 8: Cook the Cabbage Rolls: Cover the baking dish tightly with foil. Bake in a preheated oven at 375°F (190°C) for 45 minutes to 1 hour, or until the cabbage is very tender and the flavors have melded. If you prefer, you can uncover for the last 15 minutes of cooking to slightly thicken the sauce.
Step 9: Serve: Carefully remove from the oven. Let rest for a few minutes before serving hot. Garnish with fresh parsley if desired.